(a) Technical Field
The present invent relates to a valve for an engine and a method for treating the surface of the valve. More particularly, it relates to a valve for an engine and a method for treating the surface of the valve by forming a coating layer with improved abrasion resistance, heat resistance, and low friction characteristics on the surface of a base material. The present invention can, thus, ensure the quality of a valve and can further reduce abrasion of a valve guide.
(b) Background Art
Recently, in keeping with the “eco-friendly” megatrend of next-generation automobile industries, various eco-friendly vehicles are being developed aiming at reducing CO2 emission to about 50 g/km which is about 35% to 50% of the current level until 2020.
Also, vehicle manufacturers are trying to develop technologies of downsizing vehicles and improving the fuel efficiency to meet 54.5 mpg (23.2 km/L) regulations by 2025 in accordance with U.S. Corporate Average Fuel Economy (CAFE).
However, since technology such as downsizing increases loads applied to engine parts, quality reduction may result due to friction and abrasion of parts, and the duration of parts may be shortened.
These limitations have also been demonstrated in valve systems applied to vehicle engines.
As is well known, intake/exhaust valves of valve systems for vehicle engines are parts for opening/closing a combustion chamber of the engines by vertically reciprocating according to the rotary movement of a camshaft.
Intake valves open or close intake ports of the engines such that a mixture of fuel and air is supplied into a combustion chamber at an appropriate moment, and exhaust valves open or close exhaust ports such that a combustion gas is exhausted from the combustion chamber at an appropriate moment.
The intake/exhaust valves are inserted into hollow valve guides fixed to each port, and reciprocate therein. In this case, the valve guide may guide the reciprocating movement of the valve while contacting a stem part of the valve.
Referring to FIG. 1, generally, as a surface treatment method for an exhaust valve of an engine, a high frequency process is performed on a tip part 12, i.e., an upper end of a valve 10, and then a Cr plating process is performed on a stem part 11 that is in frictional contact with a valve guide.
In this case, the Cr plating is performed over the stem part 11 including the tip part 12 on which the high frequency process has been performed.
However, since the stem part 11 of the valve 10 plated with Cr produces friction with the inner circumferential surface of the valve guide, the inner circumferential surface of the valve guide may be abraded. Particularly, when the inner circumferential surface of the valve guide is abraded to a certain level or more due to longtime engine driving, the valve may move horizontally during the vertical reciprocating movement.
This horizontal motion results in a limitation in opening/closing the valve, and as abrasion continues, the movement of the valve becomes more unstable.
Thus, a method for preventing this abrasion is needed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.